Fiber-lasers and fiber MOPAs (master oscillator power amplifier systems) are finding increasing use in laser machining and laser material processing applications. In most such applications throughput is typically dependent on available output-power of the laser or MOPA.
Fiber-lasers and MOPA's typically pumped with diode-laser radiation. In early high-power lasers, radiation from a plurality of individual diode-lasers was coupled into a gain-fiber of the laser of MOPA by a corresponding plurality of pump-radiation transport-fibers fused into the cladding of the gain-fiber.
As power output requirements increased, it has been found more convenient to directly focus the output of a plurality of diode-laser bars into one or both ends of a gain-fiber of the laser or MOPA. A diode-laser bar includes a plurality of individual emitters formed in a common substrate (the “bar”). A commercially available single diode-laser bar can provide a total power output-power of between about 50 and 100 Watts. When more power is required a plurality of diode-laser bars can be physically stacked on above another. Alternatively the plurality of diode-laser bars can be mounted separately with the output beams from the beams optically stacked one above another.
Although the pump radiation absorption spectrum of certain gain-fibers, such as ytterbium-doped (Yb-doped) gain-fibers is relatively broad, it is still preferable to “wavelength-lock” at a preferred wavelength. The bars and individual emitters can have a spread of peak-gain wavelengths around a gain-bandwidth of about 30 nanometers (nm) for emitters emitting around 900 nm. The individual emitters have an emitting aperture on the order of 100 micrometers (μm) wide in a so called slow (low divergence) axis and about 1.0 μm high in a so called fast (high divergence) axis.
Typically a volume Bragg grating (VBG) is used for wavelength locking diode-laser bars. This serves essentially as a narrow-band feedback mirror providing an external cavity for all of the emitters. A problem with such an arrangement is that a special glass is required for forming a VBG. This glass cannot be optically polished to a sufficient optical quality to ensure that fed-back radiation goes back entirely to the emitters of origin. This leads to some of the power out being unlocked. The unlocked power is less absorbed by the fiber laser. This can lead to stray power causing optical damage to fiber connectors.
Another problem with a VBG is that the reflection wavelength is essentially fixed. Generally it is desirable that pump radiation is absorbed along the entire length of a gain-fiber. An ability to tune the wavelength of wavelength-locking feed-back would provide a means of adjusting the pump radiation wavelength toward the edge of an absorption-peak to provide a lower absorption and a longer propagating length or toward the center of the absorption-peak to provide a higher absorption and a shorter propagation distance. There is a need for a wavelength locking arrangement for a plurality of diode-laser bars which will provide locked-wavelength tunability and minimize unlocked radiation in the combined output of the diode-laser bars.